The major focus of this research is the relation to the neoplastic state of the commonly observed deficiency in tumor tRNA of the modified nucleoside Q. The present working hypothesis is that Q hypomodification may be a predisposing factor (perhaps one of several factors) for neoplasia. Q is unique because: (1)\it is synthesized at the polynucleotide level by the exchange of Q base with a G in the first position of certain anticodons; (2)\Q base is not synthesized by mammals but is provided by the diet; and (3)\animal cells can salvage Q base, enabling its reuse when tRNA is turned over. The enzyme that inserts queuine into tRNA is constitutive. Therefore, after the dietary availability of Q, the major variables that dictate the Q content of tRNA apparently are (1)\queuine transport into the cell and (2)\queuine salvage. We are studying both as potential sources of Q hypemodification in tumors. Recently we have shown that the Q content of tRNA in human fibroblasts is inhibited by phorbol 12,13-didecanoate (PDD). The decrease in the Q content in tRNA always preceded an increase in cell saturation density, and subsequently (with time in culture), an increase in the Q content of tRNA (to levels comparable to those in untreated cultures) preceded a decrease in saturation density. This reversal of the PDD-induced alteration in tRNA modification occurs in the continued presence of the tumor promoter, and it parallels an increased ability of the cells to salvage queuine from catabolized endogenous tRNA. Moreover, the addition of exogenous queuine concurrently with the PDD significantly inhibited the PDD-induced increase in saturation density. These results not only support a role for decreased Q content of tRNA in mediating a phenotypic change induced by a tumor promoter, but also additional data indicate that PDD inhibits queuine transport. (H)